The invention relates to coating compositions, and more particularly, to coating compositions having acetoacetate-functionalized alkyd resins.
In recent years, considerable efforts have been made by the coatings industry to develop coating formulations having little or no volatile organic compound (VOC) content. Regulations to limit the amount of VOC content of industrial coatings have encouraged research and development to explore new technologies directed at reducing solvent emissions from industrial solvent-based coatings operations such as those used for automotive parts, appliances, general metal products, furniture, and the like. However, while the move to reduced organic solvent-based compositions brings health and safety benefits, these lower VOC coating compositions must still meet or exceed the performance standards expected from solvent-based compositions.
Alkyd resins are one of the most common binders used for ambient-cure, solvent-based coatings. The resistance properties of traditional, solvent-borne alkyd resins are developed via autooxidative crosslinking of the alkyd film. Crosslinking occurs when the activated methylene groups in the unsaturated fatty acids or oils of the alkyd are oxidized in air to give hydroperoxides which subsequently decompose to generate free radicals, resulting in various types of oxidative crosslinking. This oxidative crosslinking process is commonly accelerated by adding driers such as, for example, various salts of cobalt, zirconium, calcium and manganese. However, while alkyd resins have shown, and continue to show, promise, they have relatively slow xe2x80x9cdryxe2x80x9d and/or cure times, particularly at ambient temperatures. Various modifications have been made to alkyd resins to address such concerns.
One such attempt includes polymerization of an alkyd resin with a vinyl compound, such as styrene or methyl methacrylate, via a free-radical reaction, to produce a vinyl-alkyd copolymer or a vinyl alkyd. Vinyl alkyd resins generally have a higher molecular weight and a higher Tg, producing coatings with reduced tack-free time (solvent evaporation). However, the through-dry time (oxidation of the film) of such coatings is longer, due to the decreased degree of unsaturation in the alkyd as a result of copolymerization with the vinyl compound. This problem is described in further detail in Resins for Surface Coatings, Vol.1, p.181, ed. by P. K. T. Oldring and G. Hayward, SITA Technology, London, UK, 1987, which is incorporated herein by reference. An additional drawback is that paint formulations containing vinyl alkyd resins require greater amounts of solvent, due to the increased molecular weight and Tg of the vinyl alkyd.
Various methods for the preparation of acetoacetylated coating resins have been described by J. S. Witzeman et al. in the Journal of Coatings Technology, Vol. 62, No.789, pp. 101-112 (1990). This article discloses acetoacetylation of polyester resins, and their application for baking enamels with a melamine crosslinker.
U.S. Pat. No. 5,484,849 discloses vinyl polymer compositions containing pendant acetoacetate functionality which is curable in air in the presence of a source of free radicals. The vinyl polymers are prepared by free radical polymerization (i.e., emulsion polymerization) of an acetoacetate-functional monomer (e.g., acetoacetoxyethyl methacrylate) and other acrylic monomers. The patent also discloses that an autooxidizable compound is required, to provide a source of free radicals to cure and crosslink the acetoacetate functionality. As a result of this, a portion of the acetoacetate groups could be consumed during the free radical polymerization due to the reaction between acetoacetate functionality and free radicals. This could, in turn, lead to insufficient curing upon film formation. It is therefore desirable to invent an air-dry coating system based on acetoacetyl functional polymers prepared via a non-free radical reaction.
Thus, there still exists a need in the art for a modified or functionalized alkyd resin capable of undergoing crosslinking upon film formation, which can be used to prepare ambient oxidative cure, fast-dry, and high solids coatings having a low VOC content. Ideally, such coatings would exhibit the properties and advantages of high VOC coatings.
The invention provides an acetoacetate-functionalized alkyd composition comprising an acetoacetate-functionalized alkyd resin, at least one drier, and an organic solvent. The acetoacetate-functionalized alkyd resin comprises the reaction product of (a) an alkyd resin, and (b) an alkyl acetoacetate.
The invention also relates to a method of preparing an acetoacetate-functionalized alkyd composition comprising the step of contacting an acetoacetate-functionalized alkyd resin with at least one drier in the presence of an organic solvent.